A poly lactic acid resin is a biomass polymer and therefore has been drawing attention in recent years in the context of the depletion of petroleum resources, the reduction of carbon dioxide emissions, and the like.
However, poly lactic acid itself is readily burned and thus difficult to be used for members that require flame retardancy, such as electrical and electronic applications. In addition, the poly lactic acid has a low crystallization rate and is unlikely to be crystallized by a common film forming procedure. Thus, a film composed of a resin composition containing the poly lactic acid has a problem of poor heat resistance. For example, such a film is thermally deformed at about 60° C. or more that is a glass transition temperature of the poly lactic acid and cannot keep a film shape.
For providing desired flame retardancy and heat resistance to the poly lactic acid resin, the following methods and the like have been developed.
For example, there has been developed a method of providing the flame retardancy and heat resistance by the addition of a phosphorus-containing or nitrogen-containing flame retardant into a mixture of a poly lactic acid resin and a heat resistant polymer such as a polycarbonate resin (Patent Documents 1 and 2). There has been also developed a method of providing the flame retardancy and heat resistance by heat treating, at a particular temperature, a resin composition that is obtained by the addition of a flame retardant to a mixture of a poly lactic acid resin and an amorphous resin or a low-crystalline resin, during or after injection molding to highly crystallize the poly lactic acid resin (Patent Document 3).
However, either method does not provide sufficient effect on the problem when it is used for a film or sheet. In particular, there has been developed no method that can be applied to a thin film having a thickness of less than 200 μm until now.
Commonly, the smaller thickness a film or sheet has, the more difficult it is to satisfy a standard for flame retardancy (for example, UL-94 VTM standards). To address this, a flame retardant is mixed in a larger amount. However, the flame retardant is a foreign matter to the poly lactic acid resin, and thus such a poly lactic acid resin has a problem of the reduction of the breaking strength or tear strength.
There is another problem. That is, when a resin composition containing the poly lactic acid is melted to form a film or sheet using metal rolls, the resin composition adheres to the metal rolls to interfere with the formation of the film or sheet because the resin composition has a poor releasability from the rolls.
For the heat resistance, there were reported methods for providing the heat resistance by accelerating the crystallization of the poly lactic acid resin by the following methods.
For example, there has been developed a method in which the poly lactic acid resin is subjected to, for example, melt extrusion to form a sheet and the sheet is biaxially stretched to accelerate stretching oriented crystallization (Patent Document 4). However, this method has a disadvantage that a high operating temperature largely increases heat shrink due to internal stress during the stretching. Hence, the actual operating temperature is at highest about 100° C.
A method of crystallization by the addition of a crystal nucleating agent also has a problem. That is, in common film formation, a film is cooled to a glass transition temperature or less immediately after the melt film formation in order to keep the film shape, while the film has a small thickness. Thus, the cooling rate is increased and the advantageous effect of the crystal nucleating agent is unlikely to be achieved. To address the problem, it has been developed that a heating step at 60 to 100° C. is arranged for accelerating the crystallization after the film formation step (Patent Document 5). However, this method is inefficient because a film is once cooled and solidified, and then heated again.